The success of microorganisms as human pathogens depends in part on their ability to escape intrinsic defense mechanisms or acquired immune responses of the host. One of these intrinsic defense mechanisms, the host complement system, provides a major means of control and elimination of infectious agents. Evasion of the complement system by microorganisms is therefore an essential event in the establishment of infection. The focus of this research proposal concerns an elucidation of the molecular mechanisms involved in evasion of complement mediated killing by the protozoan parasite, Trypanosoma cruzi, the etiologic agent of Chagas disease. Chagas disease is a major public health concern in rural areas of Central and South America, where it is estimated that 10 to 15 million people are currently infected. Trypomastigotes prevent complement-mediated lysis by the production of factors which interfere with the activation and amplification of classical and alternative complement components. One such factor is a 160 kilodalton, trypomastigote-specific protein which restricts alternative complement activation by binding the complement component C3b. This interaction competitively inhibits the further assembly of active enzyme complexes of the complement cascade. The 160 kilodalton protein will be further biochemically characterized with respect to its complement regulatory activity. We will use parasite purified protein and/or functional recombinant protein recovered from expression systems for these studies. The functional domain of the protein will be identified by partial proteolytic digests and C3b affinity chromatography. Fine structure epitope mapping will be carried out using deletions of the structural gene and monoclonal antibodies which block the activity of the protein. Biochemical and immunological characterizations of this protein and determination of the means whereby the parasite is protected from destruction in the bloodstream will provide critical information concerning an early step in the infectious process. Studies such as these, which define an immune evasion mechanism of the parasites, will identify specific targets for the eventual development of immune intervention strategies.